AC3

ATSC A/52a is a standard for lossy encoding of audio in digital television broadcasting in the United States. It is the same as Dolby Digital AC3.

EAC3

ATSC A/52b is an extension to A/52a and was approved for the Blu-ray and HD DVD standard. It is the same as Enhanced AC3 or EAC3 for short. EAC3 is also supposed to be backwards compatible with AC3. But that is not the case, you have to transcode somewhere in the middle of the decode process.